This commit is similar to 788e3b31,
and removes more "pure" attributes which were placed on functions that
could throw exceptions, which is illegal according to clang's
understanding of the attribute (but not according to GCC's).  GitHub
issue #58 was most likely about StorageDirectoryReader::GetInfo() and
Storage::GetInfo(), which still had "pure" attributes.

Closes #58

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Move code from ReplayGainConfig.cxx.

Move code from replay_gain_get_real_mode().

Prepare full C++ exception support in the player thread.

Previously the id was being returned only when a position wasn't
specified.

playlist_file_print() already handles stored playlists.  spl_print()
is never called with a successful result.

Prepare for moving idle events to class Partition.  Right now, it's
just a wrapper for idle_add().